It is known that road usage by vehicles continues to increase, year on year. Increased road usage causes many problems, such as increased congestion, longer travel time, higher travel costs, increased air pollution, increased accident risk, etc. In order to cope with this steady increase, solutions are required to better manage vehicle road usage. A possible solution is to construct new roads, which is unlikely to happen on a large enough scale. A further solution is to reduce traffic and/or provide alternative transportation options, neither of which is viable in most practical scenarios.
A further solution that is being widely researched and developed is the use of intelligent traffic (or transportation) systems (ITSs). The European Commission Mobility & Transport Department reported that more than one million traffic accidents have caused over 25,000 fatalities. As such, ITS has evolved to be a promising solution to improve traffic safety. ITS is being developed for a variety of applications, such as a stationary vehicle warning following an accident or vehicle problem, traffic condition warning (such as a traffic jam ahead warning), regulatory/contextual speed limits, road work warnings, detour notifications, etc. Some ITS solutions propose a communication backbone employing V2X communication (i.e. a vehicle-to-vehicle infrastructure).
In ITS, broadcasting of real-time information of cars, e.g. their positions and speed, road conditions, events and accidents, is performed in a form of beacon messages via a Vehicular Ad-hoc NETwork (VANET) built on IEEE 802.11p and a dedicated short-range communication (DSRC). The broadcast information is received from neighbouring vehicles, and enables nodes (sometimes referred to as ITS stations) to create a dynamic map of its neighbours and to some degree their operational status. This dynamic map helps to track the ITS station's neighbours and predict dangerous situations, thereby enabling various safety applications, as described in ‘Intelligent Transport Systems (ITS); Vehicular Communications; Basic Set of Applications; Definitions’ authored by the European Telecommunications Standards Institute (ETSI) Intelligent Transport Systems (ITS), Tech. Rep., 2009. It is anticipated that future autonomous vehicles will require even higher levels of safety, together with better supporting technologies in order to realize reliable safety applications.
Safety applications, such as ITS, require reliable communication in order to satisfy various technical constraints, such as minimum message rate and range. Such technical constraints need to be satisfied by all nodes/ITS stations within the ‘awareness range’ of the application. Awareness range is defined as the minimum distance between the vehicles at which the applications should predict a potential collision, such that the maneuver of vehicles would avoid a potentially catastrophic accident. In essence, an application requires ‘N’ packets to be received over a period of ‘T’ seconds from neighbours within its awareness range to ensure reliable functionality.
One known ITS station 100 is shown in FIG. 1. ITS station 100 includes a wireless transceiver integrated circuit (TRx IC) 108 that comprises a wireless transmitter and a wireless receiver connected to an antenna 102. The receiver is arranged to receive ITS messages broadcast from other local vehicles or fixed roadside units. The transmitter is arranged to broadcast ITS messages to other local vehicles or fixed roadside units.
The wireless TRx IC 108 is coupled to a baseband (BB) circuit 130, which may be of the form of a digital signal processor (DSP) and include, say, quadrature channel low pass filters (LPFs) and quadrature analog-to-digital converter (ADC) functionality. Communication between the wireless TRx IC 108 and the BB circuit 130 may use the IEEE 802.11p communication protocol. The IEEE 802.11p is an update to the IEEE 802.11 standard that adds wireless access in vehicular environments (WAVE), namely enhancements to 802.11 required to support ITS applications. This includes data exchange between high-speed vehicles and between the vehicles and the roadside infrastructure in the licensed ITS band of 5.9 GHz (5.85-5.925 GHz). IEEE 1609 is a higher layer standard based on the IEEE 802.11p. The BB circuit 130 typically performs the processing up to a data link layer (physical (PHY) layer and part of the medium access control (MAC) layer.
The system has a micro controller unit (MCU) 140 that is connected, via, say, a universal signal bus (USB) 138, to the BB circuit 130 that executes a protocol 1609 stack, and thus converts IEEE1609 messages into radio frequency (RF) signals for broadcasting. The MCU 140 is also coupled to a security circuit 150 that is used for signature generation for IEEE 1609.2 messages. ITS 100 is thereby able to receive 802.11p packets with messages from other vehicles, as well as transmit 802.11p packets with messages to other vehicles. In the current scenario 802.11 MAC would decide when a channel is free for it to broadcast the message. An ITS enabled vehicle broadcasts information to all other surrounding vehicles, which are similarly enabled. In this way, the surrounding vehicles can interrogate and process received ITS messages and build up an understanding of their surroundings vis-à-vis other vehicles or road events.
However, it is known that channel congestion is one of the most critical issues in IEEE 802.11p-based vehicular ad-hoc networks because congestion may lead to unreliability of applications. As a counter measure, ETSI proposed a mandatory Decentralized Congestion Control (DCC) framework to keep the channel load below a specific level in order to avoid such congestion. DCC algorithms are therefore focused on tuning one or more operational parameters, such as transmit power, message rate and modulation data rate, in order to avoid/limit congestion. One such algorithm tunes the message rate and transmit power according to a measured Channel Busy Percentage (CBP), which indicates a fraction of time that the channel is sensed busy by a vehicle. It is known, however, that this may have a negative impact on the application reliability. For example, a reduction of message rate will reduce the vehicle state information update rate in the VANET, thereby affecting the awareness refreshing rate of applications and thus jeopardize an application's reliability. Application reliability is strongly dependent upon an information update rate. A low update rate will delay the reaction of the application. It also reduces the chance to confirm a situation and make it less reliable for a vehicle to detect and predict a situation of other vehicles. Additionally, a safety application requires a certain message rate in order to reach its designed reliability. Hereafter, a reference to ‘application reliability’ is intended to cover one or more of various applications. In some examples, the ‘application reliability’ may be targeted towards safety applications, as they have stringent update rate (minimum message rate) and range requirements.
Furthermore, fair allocation of resources, such as message rate, transmit power, etc., is an important requirement for DCC algorithms. ‘Fair allocation’ means that vehicles experiencing similar channel loads, should be entitled to similar transmission parameters. An unfair allocation of resources may lead to a variation in range, message rate, etc., affecting application-reliability. A DCC algorithm may tune other transmission parameters, such as the modulation data rate, in order to reduce the message transmission duration in a busy channel, whilst keeping a higher message rate in order to provide better reliability even at high vehicular densities. Studies indicate that modulation data rate based congestion control strategies provide better application reliability than message rate or transmit power based congestion control strategies for various densities, although higher modulation data rate communication has range trade-off due to higher signal to interference plus noise ratio (SINR) requirements. Studies, thus far, have proposed combined message rate and transmit power strategies.
Thus, in summary, an unfair allocation of resource will cause different ITS stations with similar channel activity to have different transmission ranges, and also adversely affect applications that are in need of reliable information exchange. A more suitable, fairer, solution is desired.